Since ultrasound traversing a material is largely scattered and attenuated at a defect portion, the property of the ultrasound is different depending on whether or not the defect exists. Ultrasonic defect detection is a method for detecting a defect in a material by using the above property. In the ultrasonic defect detection, a piezoelectric device is often conventionally used as an ultrasound sensor. Since a microfracture in a material emits an acoustic emission (AE), detecting the AE allows monitoring a microfracture state in the material. Since the AE is an elastic wave in the ultrasound range, a piezoelectric device is often used as the AE sensor in the same manner as the ultrasound sensor.
Disadvantageously, the piezoelectric device, which is an electric sensor, is affected by electromagnetic interference and cannot be used in a flammable atmosphere. In recent years, Fiber Bragg Grating (hereinafter also referred to as “FBG”) has attracted attention as an ultrasound/AE sensor for solving these problems. The FBG is a kind of optical fiber sensors.
An ultrasound or AE detection system that uses the FBG as a sensor is largely divided into two types: the system that uses a laser as a light source; and the system that uses broadband light as a light source. In the case of using the laser, laser lases at a wavelength at which reflectivity of FBG reflection spectrum is largely changed. Such laser is incident on the FBG through an optical circulator. As a result, it is possible to obtain FBG reflection light intensity synchronized with an ultrasound or AE vibration received by the FBG. In the case of using the broadband light, broadband light including an FBG reflection wavelength range is incident on the FBG through an optical circulator. The FBG reflection light is incident on an optical filter with reflection property or transmission property that rapidly changes in the FBG reflection wavelength range. As a result, it is possible to obtain optical filter transmission or reflection light intensity synchronized with an ultrasound or AE vibration. Even when the optical filter is disposed between the broadband light source and the optical circulator, the same effect can be obtained. Non-Patent Document 1 written by the inventor describes an experiment in which each light source is used for ultrasound detection and ultrasonic defect detection in the FBG. Related arts include Patent Documents 1 to 5 by the inventors.
In the structure of the Fiber Bragg Grating (FBG), a core operating as a waveguide of an optical fiber has a refractive index that changes periodically in the fiber axis direction. The FBG reflects narrow-band light around a Bragg wavelength λB represented by Formula (1).
[Expression 1]λB=2nΛ  (1)
Here, n represents a refractive index and A represents a period interval (grating interval) of the change of the refractive index. When the FBG receives a temperature change or a strain change, the refractive index n and the grating interval Λ are changed. Accordingly, the Bragg wavelength λB is changed (see Formula (1)). The amounts of the Bragg wavelength change caused by the strain change and the temperature change are 1.2 pm/microstrain and 14 pm/° C. respectively in an FBG having the Bragg wavelength in 1.55 μm band generally used in a communication field. An elastic wave such as ultrasound and an AE causes a feeble vibration of about several microstrains at most. If the FBG receives ultrasound wave or an AE, the Bragg wavelength is changed by only about several pm at most. Generally, many of FBGs used to evaluate soundness of structure have a grating length of 1 to 20 mm and the reflection spectrum full width of about 1 to 2 nm at most. If the FBG receives a large temperature or strain change, the Bragg wavelength is largely changed. In a system using a laser light source, the laser wavelength may be therefore outside the reflection wavelength range of the FBG. In a system using a broadband light source, the reflection spectrum of the FBG does not cross over the wavelength range of the optical filter in which the reflection property or the transmission property is rapidly changed. In this case, the ultrasound or the AE received by the FBG cannot be detected.
One possible solution is use of a wavelength variable laser or tunable filter to control the wavelength of the laser or the wavelength at which the optical property of the optical filter changes according to the change of the Bragg wavelength of the FBG. Disadvantageously, the control cannot follow the change of the Bragg wavelength when the Bragg wavelength of the FBG rapidly changes. For this reason, the ultrasound and the AE cannot be detected. The AE in particular is caused by an instantaneous strain change that occurs when the material is broken. Thus, the generation of the AE inevitably accompanies a fast Bragg wavelength change. For this reason, it may be difficult to detect the AE in a measurement system that uses a wavelength variable laser or a tunable filter.
To solve this problem, Patent Document 3 by the inventor (“material soundness evaluation apparatus”) discloses a technique for detecting an ultrasound or an AE propagating through a test object without attaching an FBG to the test object. In a conventional detection of ultrasound or AE by the FBG, the FBG is attached to the test object. In contrast, in this technique, a portion other than an FBG in an optical fiber to which the FBG is written is in contact with the test object. The ultrasound or the AE propagating in the test object flows into the optical fiber through a contact point of the optical fiber, propagates in the optical fiber, and ultrasonically vibrates or AE-vibrates the FBG. Since the FBG is not attached to the test object, the Bragg wavelength of the FBG is no longer affected by the strain received by the test object. Nevertheless, the Bragg wavelength of the FBG is inevitably changed by temperature change. As a result, there remains a problem in the detection of the ultrasound and the AE at variable temperatures.
Patent Document 4 by the inventor (“AE/ultrasound detection system, and material monitoring apparatus and nondestructive testing apparatus including the same”) discloses an ultrasound/AE detection technique that uses two Fabry-Perot filters and that is independent of the Bragg wavelength of the FBG. In this related art, broadband light is incident on the FBG and reflected light from the FBG is incident on the two Fabry-Perot filters. The two Fabry-Perot filters have a free spectral range (FSR: interval of transmittance peak wavelengths of Fabry-Perot filter with periodic transmission property) that is substantially the same as the reflection spectrum full width of the FBG. In addition, the two Fabry-Perot filters have transmittance peak wavelengths different from each other by FSR/4. In this technique, the transmission light intensity of at least one Fabry-Perot filter varies in synchronization with the ultrasound or AE vibration received by the FBG regardless of the Bragg wavelength of the FBG. For this reason, it is possible to detect the ultrasound or the AE independently of the change of the Bragg wavelength.
There are two requirements in the ultrasound/AE detection system described in Patent Document 4: First, two Fabry-Perot filters have an FSR that is substantially the same as the reflection spectrum full width of the FBG; Second, the transmittance peak wavelengths of the two Fabry-Perot filters are shifted from each other by FSR/4. Yet, the first requirement presents difficult to accurately control the reflection spectrum full width of the FBG and the FSR of the Fabry-Perot filters in a manufacturing process of these. Thus, expensive system constituent elements may be required. Further, to shift the transmittance peak wavelengths of the two Fabry-Perot filters from each other by FSR/4 (second requirement), it is necessary to attach a temperature adjustment unit for controlling the transmittance peak wavelength to the Fabry-Perot filters or prepare a large number of Fabry-Perot filters and select two Fabry-Perot filters whose transmittance peak wavelengths are shifted from each other by FSR/4 from the prepared Fabry-Perot filters. As described above, in the technique of Patent Document 4, there are problems that the system constituent elements are expensive and the number of the system constituent elements increases.